The present invention relates to the field of synthetic polymers obtained from water-soluble monomers, or from mixtures of such monomers, and their specific application to the manufacture of a sheet of paper, paperboard or the like.
These polymers were introduced as flocculating agents some forty years ago, with molecular weights which at that time were relatively low. U.S. Pat. No. 3,325,490 (Goren) describes various gel polymers.
It is possible to use some of the Goren polymers as coagulating agents, particularly for coagulating very fine solid matter in suspension.
Modern flocculating agents have been researched in industry for at least two decades, for example those for the flocculation of solid matter in suspension in the field of water treatment, and especially in municipal sewage treatment, which are linear polymers of very high molecular weight. U.S. Pat. No. 3,557,061 may be cited as an example in this field.
European patent 0 201 237 describes a flocculation process in which a polymer material is added to water to form an aqueous composition, and is used to flocculate the solid matter in suspension in an aqueous suspension, this polymer comprising a polymer of high molecular weight that is subjected to a shearing, this shearing being carried out before or during the flocculation and the polymer being required to have certain intrinsic properties, which are indicated in this patent.
According to this document, the polymer is a polymer of high molecular weight, formed from water-soluble monomers or from a mixture of such monomers, and the polymer is subjected to shearing. The process described in this patent is characterized in that it is possible to carry out the shearing before or during the flocculation. European patent 0 201 237 further indicates that the polymer used comprises a cross-linked water-swellable polymer which it is possible to shear to an intrinsic viscosity of at least 4 dl/g. It is also indicated that the aqueous composition containing the polymer material can be a stable and homogenous composition, the shearing in this case causing an increase in the intrinsic viscosity of at least 1 dl/g.
In this document, xe2x80x9cstable and homogenousxe2x80x9d designates a polymer composition that is stable when the polymer is at full equilibrium with the water, i.e., when it has reached its ultimate degree of solubility or swelling. The composition is also homogeneous in the sense that the polymer remains uniformly dispersed throughout the composition, without having a tendency to precipitate after several days.
This document specifically describes a number of applications for water treatment, which is clearly precisely the main application intended, and coal ore treatment.
This patent also mentions, very briefly and without providing an exemplary embodiment or even any precise instructions for implementation, an application to paper or paperboard manufacturing; it merely indicates that the polymer can be added at an early stage of the pulp (fibrous mass) circulation line with a shearing along the flow line of the suspension, near the drainage stage or another water removal stage. The patent indicates that the shearing is carried out by pumping, hence by means of the xe2x80x9cfan pumpxe2x80x9d or mixing pump effectively disposed in line in paper machines.
For the other applications, and especially for water treatment, the document also indicates that it is possible to carry out the shearing on the production line, as the suspension to be flocculated approaches a centrifuge, a filter press or a belt press, or another water removal stage. It is also indicated that the shearing can be carried out during a water removal stage that is conducted under a certain shear, preferably in a centrifuge or even in a filter press or a belt press.
Hence, this document only teaches a shearing of the flocs in the mixing pump or xe2x80x9cfan pumpxe2x80x9d for the papermaking application. Moreover, it teaches that very low shear rates can be appropriate in the other applications, since filter presses and belt presses induce very low shear.
The present invention relates to a considerable improvement of this process and of the corresponding flocculating agents, with unexpected advantages in the application specific to the manufacture of a sheet of paper, paperboard or the like when operating under specific conditions, which are described below.
According to the present invention, the only flocculating agent used, which is intended to markedly improve the retention, formation, drainage and other properties of the paper or paperboard sheet thus obtained, is a cross-linked polymer or copolymer formed from suitable water-soluble monomers or mixtures of such monomers.
The process according to the invention is characterized in that the cross-linked polymer is sheared before introduction or injection into the suspension to be flocculated.
Therefore, there is no shearing of the suspension containing the polymer.
The monomers can be nonionic, but generally at least some of the monomers used to form the polymer are ionic. The monomers are usually monomers with monoethylenic unsaturation, sometimes allylic monomers, but generally vinyl monomers. These are generally acrylic or metacrylic monomers.
Suitable nonionic monomers are acrylamide, metacrylamide, N-vinyl methyl acetamide or N-vinylformamide, vinyl acetate, vinylpyrrolidone, methyl methacrylate or other methacrylates of acrylic esters, or of other esters with ethylenic unsaturation, or of other vinyl monomers that are insoluble in water such as styrene or acrylonitrile.
Suitable anionic monomers are for example sodium acrylate, sodium methacrylate, sodium itaconate, 2-acrylamido-2-methylpropane sulfonate (AMPS), the sulfopropylacrylates or sulfopropylmethacrylates, or other water-soluble forms of these polymerizable sulfonic or carboxylic acids. It is possible to use a sodium vinylsulfonate or an allylsulfonate, or a sulfomethyl acrylamide.
Suitable cationic monomers are the dialkylaminoalkyl acrylates and methacrylates, particularly dialkylaminoethyl acrylate, as well as their salts acidified or quaternized by means known to one skilled in the art, such as benzyl chloride, methyl chloride, aryl chloride, alkyl chloride, dimethyl sulfate, and even the dialkylaminoalkylalkylacrylamides or -methacrylamides, as well as their salts acidified or quaternized in a known way, for example methacrylamidopropyltrimethylammonium chloride (MAPTAC) and the Mannich products such as the quaternized dialkylaminomethylacrylamides. The alkyl groups in question are generally C1-C4 alkyl groups.
The monomers can contain hydrophobic groups, for example as described in European patent 0 172 723, and in certain cases allylic ether monomers could be preferred.
The cross-linking can be carried out during or after the polymerization, for example by reaction of two soluble polymers having counter-ions, or by reaction on formaldehyde or a polyvalent metal compound. Often the cross-linking is carried out during the polymerization by addition of a cross-linking agent, arid this method is clearly preferred according to the invention. These processes for polymerization with cross-linking are known.
The cross-linking agents that can be incorporated include ionic cross-linking agents such as polyvalent metal salts, formaldehyde, glyoxal, or preferably, covalent cross-linking agents that will copolymerize with the monomers, preferably monomers with diethylenic unsaturation (like the family of diacrylate esters such as the diacrylates of polyethylene glycol PEG) or polyethylenic unsaturation, of the type classically used for the cross-linking of water-soluble polymers, and particularly methylenebisacrylamide (MBA), or any of the other known acrylic cross-linking agents.
The quantity of cross-linking agents, and particularly of methylenebisacrylamide (MBA), that can be incorporated according to the invention is generally on the order of 5 to 100 ppm, preferably 5 to 40 ppm, and according to the best method, around 20 ppm of cross-linking agent/active material (polymer).
For details of the above, please refer to the content of European patent 0 201 237, which is incorporated herein by reference.
The polymers that are usable according to the invention can be prepared by a low concentration aqueous solution polymerization method, but the invention primarily relates to a reverse phase emulsion polymerization, that is, a water-in-oil emulsion polymerization.
Also known in the prior art are systems of retention agents for the manufacture of a sheet of paper, paperboard or the like, which comprise a combination of two retention agents, generally a main retention agent and a secondary retention agent. These are called xe2x80x9cdualxe2x80x9d systems.
Thus, in U.S. Pat. No. 4,753,710, it is recommended to use a linear acrylic polymer of high molecular weight as the main retention agent, which is added to the fibrous mass, followed by an intense shearing, particularly in the mixing pump or xe2x80x9cfan pump,xe2x80x9d then an addition of bentonite (which is a swelling clay) as the secondary retention agent. This document neither suggests nor describes any shearing of the polymer itself before introduction into the suspension to be flocculated.
Also known in the prior art are cross-linked flocculating agents as described, for example, in European patent 0 202 780, primarily for the treatment of water, and secondarily for paper. It is important to note that it uses a cross-linked product which is added to the suspension to be flocculated, the flocs then being sheared during the paper manufacturing process, i.e., sheared in and at the same time as the paper pulp. The flocs are then transformed into flocs that are smaller and more shear resistant, therefore more tenacious. This document neither suggests nor describes any shearing of the polymer itself before introduction into the suspension to be flocculated.
Thus, according to the techniques of the prior art relative to papermaking applications, between the flocculating agent and the fibrous mass of pulp, flocs of fairly large size are formed, and are then sheared so as to form flocs which, in the documents cited, are said to be smaller and more tenacious.
Moreover, the systems of the prior art of the dual system type require the use of two retention components, and specifically the use of bentonite, which is a difficult product to use in industry, if only due to the environmental problems it causes. This bentonite is necessary to induce the reassembly of the sheared flocs, otherwise the latter will not stick to the drainage wire of the paper. Systems of this type can be classified as xe2x80x9cmicroparticulate,xe2x80x9d and they comprise at least two retention agents.
The xe2x80x9cdualxe2x80x9d systems of the prior art were essentially composed of linear polymers with an addition of bentonite, or of a branched polyacrylamide or a starch, with an addition of colloidal silica, this last component being extremely expensive.
A known improvement of these processes is described in French patent 95 13 051 in the name of the Applicant, which relates to a dual system based on a polymer of the linear or branched polyacrylamide type and kaolin, kaolin being a non-swelling clay that does not have the drawbacks of bentonite, the kaolin being pre-treated in a preferred embodiment.
On the other hand, according to the present invention, only one retention agent is used, preferably in the form of a cross-linked reverse phase water-in-oil emulsion sheared before its injection, which leads directly to microflocs without going through the shearing of larger flocs involving the fibrous mass.
According to the invention, and without intending to be limited by any one theory, the Applicant in effect maintains that a microflocculation occurs directly as a result of the intense shearing carried out on the polymer itself before its injection into the fibrous mass of pulp, which is quite a different (and unexpected) process than reducing the size of large flocs (involving the fibrous mass) into smaller, more tenacious flocs, and which results in unforeseen improvements in the properties of the paper or paperboard sheet.
According to the invention, this xe2x80x9cmicroflocculationxe2x80x9d also avoids the presence of bentonite or another second xe2x80x9cdualxe2x80x9d retention agent.
According to the invention, the notable constraints and problems linked to the control and optimization of the flocculation parameters of a dual system, including the precise choice of the respective introduction points of the two reagents, are also avoided.
Reverse phase emulsion polymerization is quite well known to one skilled in the art.
It is noted that, contrary to certain aspects of the teaching of European patent 0 201 237, a shearing under the flow line conditions described in this document with a brief reference to paper manufacturing, page 6, lines 3-4, absolutely does not lead to the results of the invention.
For example, a shearing in a pump of the xe2x80x9cfan pumpxe2x80x9d type does not produce the anticipated result. Specifically, and again in reference to an extremely brief mention of xe2x80x9cpaper and paperboard production,xe2x80x9d the introduction according to European patent 0 201 237 of the retention agent along the xe2x80x9cflow linexe2x80x9d of the process, near a drainage stage or another water removal stage, absolutely does not work.
On the other hand, it has been discovered according to the invention that, for the application related to the manufacture of a sheet of paper, paperboard or the like, it is essential to carry out an intense shearing before the injection of the cross-linked polymer into the paper pulp or fibrous mass prior to being flocculated.
The cross-linked polymer, pre-sheared according to the invention, is injected or introduced into the paper pulp (or fibrous mass to be flocculated), which is more or less diluted in accordance with the experience of one skilled in the art, and generally into the diluted paper pulp or xe2x80x9cthin stock,xe2x80x9d i.e., a pulp diluted to about 0.7%-1.5% solid matter such as cellulose fibers, possible fillers, and various additives commonly used in paper manufacturing.
According to a variant of the invention with a fractionated introduction, some of the cross-linked polymer, sheared according to the invention, is introduced at the level of the stage for preparing the xe2x80x9cthick stockxe2x80x9d with about 5% or more solid matter, or even at the level of the preparation of the thick stock.
According to a variant of the invention, it is possible to use a reverse phase emulsion of the polymer, or even the powder obtained from the emulsion by means of a known drying technique such as, for example, xe2x80x9cspray-drying,xe2x80x9d solvent precipitation, or agglomeration (PEG) and grinding, (on this subject, see also the prior art, such as U.S. Pat. No. 5,696,228, WO 97/48 755 (U.S. Ser. No. 08/668,288) WO 97/48 750, WO 97/48 732, WO 97/34 945, WO 96/10589, U.S. Pat. Nos. 5,346,986, 5,684,107, EP 0 412 388, EP 0 238 050, U.S. Pat. No. 4,873,299, EP 0 742 231, WO 90/08789 or EP 0 224 923) which is redissolved in water, sheared, then used like an emulsion.
This variant is quite advantageous since the dried product according to the invention behaves substantially like the emulsion, and this variant therefore provides a method for using dry products having the advantages of an emulsion, which it is not always possible to prepare by direct polymerization in the aqueous phase, in gel form or in solution.
According to the invention, it is preferable to use the reverse phase emulsion of the cross-linked polymer, or even the cross-linked polymer in solution as obtained through the redissolution of a powder as described above, with shearing prior to the injection into the pulp, of course.
Given below is a comparative example which shows that if the shearing of the fibrous mass is carried out (i.e., after the addition of the polymer) in a paper application, no retention is obtained. Without intending to be limited by any one theory, the Applicant maintains that this is due to the fact that the cationic charge is not released.
According to the invention, laboratory shearing tests can be conducted, with a concentration on the order of 3-5 to 10-15 g of active material (i.e., the polymer) per liter, preferably between 5 and 10 g/l, in a piece of equipment known as an xe2x80x9cUltra Turraxxe2x80x9d (TM), for example at 10,000 rpm or in a household mixer of the xe2x80x9cMoulinexxe2x80x9d (TM) type, substantially at the same magnitude of rotation speed, for a duration that can last between 15-30 seconds and 2-5 minutes.
In the industry, there is existing equipment suitable for implementing the invention, for example high-pressure pumps or turbines, which are not referred to by the theoretical example of the document EP 0 201 237.
One skilled in the art will naturally know all the equipment that makes it possible to carry out an intense shearing on the polymer emulsion, diluted to an appropriate value as described below, without being limited to the above examples.
For the generalities of the production of a pulp for paper, paperboard or the like, as well as a list of the additives, fillers, etc., that are well known, it would be useful for one skilled in the art to refer to U.S. Pat. No. 4,753,710, whose teaching is incorporated herein by reference.
A normal dosage of the agent according to the invention is such that it results in about 100 to 500 g of active material (polymer) per ton of fibrous matter to be processed.
According to the invention, it is possible to use a polymer having an intrinsic viscosity i.v. as low as 1 to 3, which becomes an intrinsic viscosity as high as 3-7 or 8 after the application of the shear.
According to the invention, using an optimization within the scope of one skilled in the art, an ion regain (IR as defined in European patent 0 201 237) of 40 to 50% is obtained, which can reach at least 60 or 70%, and even more, up to values greater or far greater than 100%.
Moreover, it is possible to adjust the shearing so as to favor, for the first time in this industry, one property of the paper over another, for example to promote retention slightly more than formation or drainage, or vice versa, or any of the various possible combinations, as will be seen by reading the examples that follow.
Moreover, the system according to the invention is not expensive, and consequently it combines all of the advantages of the linear or cross-linked single-product systems with floc shearing and of the xe2x80x9cdualxe2x80x9d systems with two retention agents and also with floc shearing.
The following examples illustrate the invention without limiting its scope.
The examples of the production of the polymer are followed by a Table (xe2x80x9cExample 1, 2 or 3xe2x80x9d) indicating the properties of the polymer obtained, and by a two-part Table (numbered No. 1, No. 2 and No. 3 with reference to the product in the preceding tables) indicating in one part the applicable test conditions and in the other part the results relative to retention, drainage and formation, and other analogous properties.
These two-part tables make it possible to plot the drainage, retention or turbidity graphs that are appended in the form of FIGS. 1 through 8.
The abbreviations have the meanings indicated below.
RET=type of addition of the solution of the emulsion according to the invention, sheared before injection
DOS=dosage of the retention agent according to the invention, in % agent/dry pulp
TUR=turbidity
ChM=mineral filler %/dry pulp
Ash=weight of ash in grams and in %, respectively
MES=matter in suspension
GCC=ground Ca CO3 
CSF=CSF drainage
GB0=basis weight 80 g
UX=shearing time in the xe2x80x9cUltra Turraxxe2x80x9d (TM), in seconds
Pds=weight of the sheet, in grams
X designates a xe2x80x9cfirst passxe2x80x9d measurement